Wells are drilled on land and in marine environments for a variety of exploratory and extractive purposes. Due to the variety of purposes, the conditions experienced while producing the wells also vary greatly. The particular conditions include changes in temperature, pressure, subterranean fluids, and formations, among other variables. The equipment used, including the configuration of the bottomhole assembly, will be affected by subsurface conditions. Managed Pressure Drilling (“MPD”) is used to ensure the pressure within the wellbore is maintained within predetermined limits relative to the surrounding formation pressure. The formation pressure may change during drilling of the wellbore. The applied fluid pressure by the drilling system is increased or decreased to keep the wellbore pressure within the desired limits.
A drilling system includes a drilling rig outside of the wellbore and a drill string with a bottomhole assembly near or at the bottom of the wellbore. The drilling rig often includes a platform, a rotating table, a kelly, pressure control devices such as one or more blowout preventers, a rotating control device (“RCD”), and a choke. The drilling rig stabilizes and controls the upper end of the drill string, which extends downward. The drill string includes drill pipe in segments mated together at threaded joints. The drill pipe provides force transmission and a fluid conduit down to the bottomhole assembly at the end of the drill pipe. The bottom of the drill pipe is connected to the bottomhole assembly. The bottomhole assembly has a variety of equipment and modules that enable operators to monitor and control the drilling progress. The bottomhole assembly includes components such as a drill bit, a drill motor, measurement-while-drilling equipment, logging-while-drilling equipment, and a drill collar.
During drilling, a drilling fluid is pumped from the drilling rig down the fluid conduit within the drill pipe to the bottomhole assembly. The drilling fluid passes through a fluid conduit extending through the bottomhole assembly and passes through the drill bit, producing a positive pressure at the bottom of the wellbore. The composition of the drilling fluid also changes depending on the conditions of the formation through which the wellbore will extend. Generally, however, the drilling fluid is used to lubricate and cool the drill bit while also removing drill cuttings from the wellbore. The drilling fluid flows back up the wellbore in annular gap around the drill string, carrying drill cuttings that are suspended in the drilling fluid.